Look out for the date of what you read. Anything, what, before April 2011 is going to give you a much different BTC income due to the lower difficulty at the time. From April-June? the $$ income changes drastically.

I've seen somewhere posted, that it's possible to make 50BTC with one 6990 within 3 days.. So know I understand that post was fake..

Or it is outdated...there's this concept called "difficulty increase" that you should understand.

Or you could explain it to the new guy:

Bitcoin tries to spit out 1 block every 10 minutes. Doesn't matter if there is one guy running one CPU, or a million 6990's on the network. The more power the network has, the higher the difficulty.

When bitcoin first launched, there was only CPU mining - thus a very low difficulty. GPU mining increased the difficulty greatly. So there was a period when the first GPU miners made a great deal more, but eventually more and more people upgraded to GPU mining.

The second chart compares single-hash MD5 brute forcers running on as many of the fastest GPUs they each support. Note that 8 x HD 5870 has not been tested with any of the tools because it is unknown if this configuration is supported: